Internal combustion engine exhaust emissions, and especially diesel engine exhaust emissions, have recently come under scrutiny with the advent of stricter regulations, both in the U.S. and abroad. While diesel engines are known to be more economical to run than spark-ignited engines, diesel engines inherently suffer disadvantages in the area of emissions. For example, in a diesel engine, fuel is injected during the compression stroke, as opposed to during the intake stroke in a spark-ignited engine. As a result, a diesel engine has less time to thoroughly mix the air and fuel before ignition occurs. The consequence is that diesel engine exhaust typically contains incompletely burned fuel known as particulate matter, or “soot”.
It must be noted that other types of internal combustion engine ignition processes are also known to produce soot in the exhaust, for example, direct injection gasoline engines. Hence, the problem addressed by the present invention is broader than just diesel exhaust soot, although that is the largest application for the present invention at the present time. For this reason, the terms “catalytic diesel particulate filter (CDPF)” and “diesel particulate filter (DPF)” as used herein should not be limited to diesel engines but rather must be taken to mean a particulate filter for capturing soot particles in any internal combustion engine exhaust.
It is known to use catalytic particulate filters which physically trap soot particulates. However, such particulate filters progressively load up with accumulated soot and therefore must be repeatedly regenerated by burning off the trapped particulates, typically on a fixed schedule and by fuel and oxygen enrichment of the exhaust stream entering the CDPF and catalytically ignited in an integral diesel oxygen catalyst (DOC).
Typically, prior art regeneration systems are temperature based with the primary filter protection strategy being limitation of the quantity of soot allowed to accumulate. As shown below, such a strategy can under-utilize the filter capacity by frequent regeneration on a conservative schedule and thus result in a penalty in fuel economy.
A currently challenging durability issue in the CDPF art is cracking or melting of a CDPF substrate due to large temperature excursions within the bed of the filter during regeneration, especially when using an economical filter such as a cordierite monolith. These temperature excursions are caused by the exothermic reaction of carbon and oxygen due to the combined effects of the mass loading and distribution of wet volatile and dry soot within the CDPF, the operating condition of the engine, and the exhaust gas temperature and flow rate through the CDPF. Diesel engine exhaust temperatures are normally in the range of 200-500° C., depending in part on the amount of exhaust gas recirculation, throttle plate position (MVRV—Manifold Vacuum Regulator Valve) and fueling. These engine control parameters, in combination with the manipulation of both fuel quantity and timing in the main fueling and post fueling events, may be used to increase exhaust gas temperature in the range of 500-700° C. as an effective means of initiating a regeneration event and as a means of controlling exhaust gas temperatures supplied to the CDPF during the regeneration process. During regenerative events, when the exhaust gas contains sufficient available oxygen to support the O2 transport process (typically, 5-11%) and an adequate (actual mass depends upon wet/dry soot ratio and total mass) non-homogeneous distribution of wet and dry soot is resident within the CDPF, a highly non-uniform uncontrolled reaction can occur within the CDPF. This rapid, non-uniform reduction of wet and dry soot within the CDPF under various conditions of engine load and exhaust gas temperature and flow may result in excessive thermal gradients and peak monolith temperatures that exceed the material capabilities of the substrate material. This combination of events (rapid oxidation and inadequate heat transfer due insufficient exhaust gas flow) can result in excessive filter temperature and/or temperature gradients, resulting in substrate failure.
A factor not recognized in prior art CDPF regeneration is the relative combustibility difference between “wet” soot and “dry” soot, both of which can be present in a CDPF. By “wet soot” is meant soot particles coated with residual diesel fuel, such as may be generated during periods of high engine load but low engine speed, for example when pulling a heavy vehicle load up a substantial incline in a relatively high gear. Conversely, dry soot may be generated during periods of low engine load and high engine speed, such as at constant highway vehicle speeds. Wet soot burns substantially hotter that dry soot during catalytic regeneration. Indeed, wet soot is inherently rich in hydrocarbons that can explosively ignite, either spontaneously or when regeneration is started, and create an intense exothermic reaction within the CDPF in which temperatures can rise rapidly and uncontrollably (“flash-over”). Further, such intense combustion may occur nonuniformly over a CDPF, creating thermal stresses that can cause cracking or melting of the monolith, resulting in filter failure. Such flash-over is analogous to a creosote fire in a wood stove or fireplace chimney flue.
U.S. Pat. No. 6,735,941 B2 discloses a method for calculating the total soot mass accumulated in a CDPF by measuring differential pressure across the CDPF. This method does not recognize the functional (combustibility) difference between wet soot and dry soot; does not determine the percentage of total soot that is wet soot; and does not provide a strategy for burning off the wet soot in a controlled manner before completing oxidation of the dry soot, to protect against thermal damage to a CDPF.
What is needed in the art is a method for continuously calculating the total soot load and the wet soot fraction of the soot load in a CDPF and determining a relative Combustibility Index for the overall soot content.
It is a principal object of the present invention to prevent damage to a CDPF substrate by overheating during regeneration thereof, by continuously calculating a Combustibility Index for the soot load within a CDPF.
It is a further object of the present invention to improve engine fuel economy by conducting CDPF regeneration only when needed, as indicated by the Combustibility Index, rather than on a fixed schedule.